This invention relates to RF load and source pull testing of low noise as well as medium and high-power RF transistors and amplifiers.
Modern design of low noise or high-power RF amplifiers and mixers, used in various communication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient for the transistors, which operate either as very low noise or as highly non-linear devices, close to power saturation, to be described using linear or non-linear numeric models. A popular method for testing and characterizing such microwave components (transistors) in the non-linear region of operation is “load pull” or “source pull” (see ref. 1). Load/source pull is a measurement technique employing microwave impedance tuners 2, 4 and other microwave test equipment (FIG. 1), such as signal source 1, source (input) 2 and load (output) 4 tuner, input and output 5 power meter and test fixture 3 which holds the DUT. The tuners and equipment are controlled by a computer 6 via digital cables 7, 8, 9. The microwave impedance tuners are devices which allow manipulating the RF impedance presented to the Device Under Test (DUT, or transistor) to test (see ref. 2); this document refers hence to “impedance tuners”, in order to make a clear distinction to “tuned receivers (radios)”, popularly called elsewhere also “tuners” because of the included tuning circuits.
Electro-mechanical impedance tuners (FIGS. 2 and 3) in the microwave and low millimeter-wave frequency range between 100 MHz and 70 GHz are using the slide-screw concept and include a slabline 24, 30 with a test port 35 and an idle port 36, a center conductor 23, 31 and two mobile carriages 33 which carry vertical motors 37 and vertical axes 34, which control simultaneously the vertical position of reflective probes 32, see ref. 4. The carriages are moved horizontally by additional motors (not shown) and gear. The signal enters into one port 35 and exits from the other 36. In load pull, the test port is the one where the signal enters, in source pull the test port is the one where the signal exits. The entire mechanism is, typically, integrated in a solid housing, since mechanical precision is of highest importance. Millimeter-waves occupy the frequency spectrum from approximately 30 GHz to 300 GHz. The wavelength (λ) is in the 10 mm to 1 mm range. The tuners in the present embodiment operate from 20 to 70 GHz, corresponding to wavelengths between 15 and 4.6 mm, though the concept of multiple slug tuning probes has been demonstrated for frequencies as low as 1 GHz. The only structural limitation is the required horizontal travel of the tuning probe. A slide screw tuner must allow horizontal travel of one half of a wavelength (λ/2), or, in the case of Fmin=20 GHz, at least 7.5 mm, in order to allow for a 360° rotation of the created reflection factor (the reflection phase is double the transmission phase). The required free horizontal travel is inverse proportional to the minimum frequency of operation (at 10 GHz it is 15 mm at 1 GHz it is 150 mm, etc..)
The typical configuration of the reflective probe inside the slabline is shown in FIG. 2: a number of parallel reflective tuning elements 21 also called “tuning” probes or slugs, are inserted into the slotted transmission airline (slabline) 24 and coupled capacitively with the center conductor 23 to an adjustable degree, ranging from very weak (when the probe is withdrawn-Top position) to very strong (when the probe is very close (within electric discharge—or Corona) to the center conductor (Bottom position); it must be pointed that capacitive “tuning” probes are different from “sampling” probes, which are loosely coupled with the center conductor; when the tuning probes move vertically 26 between the “top position” and the “bottom position”, approach the center conductor 23 of the slabline 24 and are moved along the axis 25 of the slabline, they alter the amplitude and phase of the reflection factors seen at the slabline ports, covering parts or the totality of the Smith chart (the normalized reflection factor area). The relation between reflection factor and impedance is given by GAMMA=|GAMMA|*exp(jΦ)=(Z−Zo)/(Z+Zo), where Z is the complex impedance Z=R+jX and Zo is the characteristic impedance. A typical value used for Zo is 50Ω (Ohm) (see ref. 3). In a 50Ω test system (i.e. when the tuner is terminated at both ports with 50Ω), GAMMA is equal to the first element of the tuner two-port s-parameter matrix: GAMMA=S11, also expressed as Standing Wave Voltage Ratio VSWR, whereby VSWR=(1+|GAMMA|)/(1−|GAMMA|.
Up to now such metallic tuning probes (slugs) (FIG. 2) have been made in a cubical form 21 and FIG. 6A with a concave bottom, which allows capturing, when approaching the center conductor 23, the electric field on the sides, between the center conductor and the slabline walls, because the electric field is concentrated in the narrowest space between the center conductor and the ground planes of the slabline. This field capturing allows creating high and controllable reflection factors. Disc formed rotating probes (see ref. 5 and ref. 7) are circular and rotate eccentrically, or can be oval, or elliptical and rotate centered or eccentrically. The distinct beneficial feature of rotating disc-probes is that the coupling factor (or the penetration of the disc body into the slabline) is controllable through the angle of rotation only. This eliminates the need for cumbersome vertical axis structures and a shared slabline configuration (see ref. 6) for reduced tuner length.
The RF transmission media between the reference plane of the tuning probe of the tuner and the tip of the wafer probe, introduce, especially at millimeter-wave frequencies, considerable insertion loss (see ref. 9), which reduces the tuning range of the tuner and by consequence the capacity of the tuner to conjugate-match many high-power/low-impedance transistors. It is therefore of primary importance to increase the tuning range of the tuners. Using two or more probes in a pre-matching scheme allows higher reflection factor (see ref. 3 and ref. 4). The herein newly disclosed “notched” rotating disc tuning probes allow doing this efficiently.